This invention relates to a process for making shaped articles of a thermoplastic resin having a microcellular structure, which articles are useful in diverse applications, including, i.a., vascular prosthetic devices, dialysis membranes, organoids (cellular polymer systems which deliver proteins or hormones such as, for example, insulin to diseased individuals), controlled drug release devices and other devices requiring controlled diffusion of water-soluble compounds.
Articles made from a variety of resins having cellular structure are known. The resin cells may be either open and interconnected or closed. In either case, the resin density is less than the density of the same noncellular resin.
Methods for producing thermoplastic articles having densities lower than the densities of the thermoplastic materials used to fabricate the articles include, i.a.. the use of foaming agents that release gases that expand the thermoplastic materials at their normal processing temperatures; the use of thermoplastic materials containing liquids or solids, or both, that can be removed by extraction or dissolution; and the technique of stretching thermoplastic films containing liquids or solids, or both, to produce interfacial voids, followed by extraction or dissolution. The methods that depend on extraction or dissolution require the formation of an interconnecting network of pores that allows removal of the dissolved liquids or solids.
The density value of such articles having cellular structure generally depends on the volume fraction of the material removed by extraction or dissolution. For example, if the thermoplastic article contains 50% by volume of extractable material, and the density of the starting thermoplastic polymer is 1 g/cm.sup.3, then the density of the extracted, cellular, thermoplastic article would be about 0.5 g/cm.sup.3. Density values lower than those predicted from volume fractions are obtained if voids are produced at the interface between the extractable solid and the thermoplastic matrix during fabrication or by stretching after fabrication.
It is desirable in certain applications, such as, for example, vascular grafts and organoids to use light but strong materials, which in such cases must also be biocompatible, but should not be porous to the extent of leaking blood or other fluids flowing therethrough. Accordingly, it would be advantageous to be able to make such articles from resins having a closed-cell or nearly closed-cell structure rather than a porous, open-cell structure.